BIOMASS BURNING EVENTS AND LANDSCAPE DENUDATION RECORDED BY THE DISTRIBUTION AND ISOTOPIC COMPOSITION OF POLYCYCLIC AROMATIC HYDROCARBONS IN LATE CRETACEOUS STRATA FROM SOUTHEASTERN ARIZONA

The lacustrine stratigraphic framework of the Late Cretaceous Fort Crittenden Formation reflects balanced to over-filled conditions during the development of icehouse conditions within the western interior of the U.S. Organic carbon contents, abundance of extractable organic matter and the relative concentrations of saturated and aromatic hydrocarbons are facies controlled. Carbon isotopic values of bulk organic matter range from –22.84 to –25.61‰ reflecting variations in productivity, provenance of organic matter, and episodes of anoxia and dysoxia. The saturated hydrocarbons are typical of lacustrine deposits; they show a smooth distribution of n-C₁₂ through n-C₃₉ alkanes, low or trace amounts of pristane and phytane, and a high relative abundance of branched and cyclic alkanes. The polycyclic aromatic hydrocarbons (PAH) include components of both diagenetic (e.g., phenanthrene, methylphenanthrenes, and chrysene) and pyrolytic (e.g., fluoranthene, pyrene, benzo[e]pyrene, benzo[ghi]pyrene, and coronene) origin. Vitrinite reflectance range from 0.5 – 0.9 based on equivalence to values of the methylphenanthrene index and are consistent with low to moderate thermal maturity. The occurrences of diagnostic pyrolitic PAH indicate the prevalence of wildfires or peat fires in the watershed. Landscape denudation following wildfires would then transport these components into the lacustrine system. Turbidites contain relatively large abundances of branched and cyclic hydrocarbons and pyrolitic components. Preliminary compound specific isotopic data for the n-alkanes and PAH in a given sample suggest it should be possible to decipher the net contributions of terrestrial, aquatic, pyrolytic and diagenetic d¹³C to the bulk d¹³Corg. If residual organic matter is depleted in ¹²C during biomass burning, then the pyrolytic compounds may record a d¹³C value that is distinct from diagenetic counterparts.